diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -607,21 +607,23 @@
 
         return {"snapshot:add": len(snapshots)}
 
-    def snapshot_missing(self, snapshots):
+    def snapshot_missing(self, snapshots: List[Sha1Git]) -> Iterable[Sha1Git]:
         return self._cql_runner.snapshot_missing(snapshots)
 
-    def snapshot_get(self, snapshot_id):
+    def snapshot_get(self, snapshot_id: Sha1Git) -> Optional[Dict[str, Any]]:
         return self.snapshot_get_branches(snapshot_id)
 
-    def snapshot_get_by_origin_visit(self, origin, visit):
+    def snapshot_get_by_origin_visit(
+        self, origin: str, visit: int
+    ) -> Optional[Dict[str, Any]]:
         visit_status = self.origin_visit_status_get_latest(
             origin, visit, require_snapshot=True
         )
-        if not visit_status:
-            return None
-        return self.snapshot_get(visit_status.snapshot)
+        if visit_status and visit_status.snapshot:
+            return self.snapshot_get(visit_status.snapshot)
+        return None
 
-    def snapshot_count_branches(self, snapshot_id):
+    def snapshot_count_branches(self, snapshot_id: Sha1Git) -> Optional[Dict[str, int]]:
         if self._cql_runner.snapshot_missing([snapshot_id]):
             # Makes sure we don't fetch branches for a snapshot that is
             # being added.
@@ -635,14 +637,18 @@
         return counts
 
     def snapshot_get_branches(
-        self, snapshot_id, branches_from=b"", branches_count=1000, target_types=None
-    ):
+        self,
+        snapshot_id: Sha1Git,
+        branches_from: bytes = b"",
+        branches_count: int = 1000,
+        target_types: Optional[List[str]] = None,
+    ) -> Optional[Dict[str, Any]]:
         if self._cql_runner.snapshot_missing([snapshot_id]):
             # Makes sure we don't fetch branches for a snapshot that is
             # being added.
             return None
 
-        branches = []
+        branches: List = []
         while len(branches) < branches_count + 1:
             new_branches = list(
                 self._cql_runner.snapshot_branch_get(
@@ -675,7 +681,7 @@
         else:
             last_branch = None
 
-        branches = {
+        branches_d = {
             branch.name: {"target": branch.target, "target_type": branch.target_type,}
             if branch.target
             else None
@@ -684,7 +690,7 @@
 
         return {
             "id": snapshot_id,
-            "branches": branches,
+            "branches": branches_d,
             "next_branch": last_branch,
         }
 
diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -608,32 +608,34 @@
 
         return {"snapshot:add": count}
 
-    def snapshot_missing(self, snapshots):
+    def snapshot_missing(self, snapshots: List[Sha1Git]) -> Iterable[Sha1Git]:
         for id in snapshots:
             if id not in self._snapshots:
                 yield id
 
-    def snapshot_get(self, snapshot_id):
+    def snapshot_get(self, snapshot_id: Sha1Git) -> Optional[Dict[str, Any]]:
         return self.snapshot_get_branches(snapshot_id)
 
-    def snapshot_get_by_origin_visit(self, origin, visit):
+    def snapshot_get_by_origin_visit(
+        self, origin: str, visit: int
+    ) -> Optional[Dict[str, Any]]:
         origin_url = self._get_origin_url(origin)
         if not origin_url:
-            return
+            return None
 
         if origin_url not in self._origins or visit > len(
             self._origin_visits[origin_url]
         ):
             return None
 
-        visit = self._origin_visit_get_updated(origin_url, visit)
-        snapshot_id = visit["snapshot"]
+        visit_d = self._origin_visit_get_updated(origin_url, visit)
+        snapshot_id = visit_d["snapshot"]
         if snapshot_id:
             return self.snapshot_get(snapshot_id)
         else:
             return None
 
-    def snapshot_count_branches(self, snapshot_id):
+    def snapshot_count_branches(self, snapshot_id: Sha1Git) -> Optional[Dict[str, int]]:
         snapshot = self._snapshots[snapshot_id]
         return collections.Counter(
             branch.target_type.value if branch else None
@@ -641,8 +643,12 @@
         )
 
     def snapshot_get_branches(
-        self, snapshot_id, branches_from=b"", branches_count=1000, target_types=None
-    ):
+        self,
+        snapshot_id: Sha1Git,
+        branches_from: bytes = b"",
+        branches_count: int = 1000,
+        target_types: Optional[List[str]] = None,
+    ) -> Optional[Dict[str, Any]]:
         snapshot = self._snapshots.get(snapshot_id)
         if snapshot is None:
             return None
@@ -651,7 +657,7 @@
         from_index = bisect.bisect_left(sorted_branch_names, branches_from)
         if target_types:
             next_branch = None
-            branches = {}
+            branches: Dict = {}
             for (branch_name, branch) in sorted_branches:
                 if branch_name in sorted_branch_names[from_index:]:
                     if branch and branch.target_type.value in target_types:
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -677,11 +677,11 @@
         ...
 
     @remote_api_endpoint("snapshot/missing")
-    def snapshot_missing(self, snapshots):
+    def snapshot_missing(self, snapshots: List[Sha1Git]) -> Iterable[Sha1Git]:
         """List snapshots missing from storage
 
         Args:
-            snapshots (iterable): an iterable of snapshot ids
+            snapshots: snapshot ids
 
         Yields:
             missing snapshot ids
@@ -690,7 +690,7 @@
         ...
 
     @remote_api_endpoint("snapshot")
-    def snapshot_get(self, snapshot_id):
+    def snapshot_get(self, snapshot_id: Sha1Git) -> Optional[Dict[str, Any]]:
         """Get the content, possibly partial, of a snapshot with the given id
 
         The branches of the snapshot are iterated in the lexicographical
@@ -702,7 +702,8 @@
             should be used instead.
 
         Args:
-            snapshot_id (bytes): identifier of the snapshot
+            snapshot_id: snapshot identifier
+
         Returns:
             dict: a dict with three keys:
                 * **id**: identifier of the snapshot
@@ -715,7 +716,9 @@
         ...
 
     @remote_api_endpoint("snapshot/by_origin_visit")
-    def snapshot_get_by_origin_visit(self, origin, visit):
+    def snapshot_get_by_origin_visit(
+        self, origin: str, visit: int
+    ) -> Optional[Dict[str, Any]]:
         """Get the content, possibly partial, of a snapshot for the given origin visit
 
         The branches of the snapshot are iterated in the lexicographical
@@ -727,8 +730,9 @@
             should be used instead.
 
         Args:
-            origin (int): the origin identifier
-            visit (int): the visit identifier
+            origin: origin identifier (url)
+            visit: the visit identifier
+
         Returns:
             dict: None if the snapshot does not exist;
               a dict with three keys otherwise:
@@ -743,37 +747,43 @@
         ...
 
     @remote_api_endpoint("snapshot/count_branches")
-    def snapshot_count_branches(self, snapshot_id):
+    def snapshot_count_branches(self, snapshot_id: Sha1Git) -> Optional[Dict[str, int]]:
         """Count the number of branches in the snapshot with the given id
 
         Args:
-            snapshot_id (bytes): identifier of the snapshot
+            snapshot_id: snapshot identifier
 
         Returns:
-            dict: A dict whose keys are the target types of branches and
-            values their corresponding amount
+            A dict whose keys are the target types of branches and values their
+            corresponding amount
+
         """
         ...
 
     @remote_api_endpoint("snapshot/get_branches")
     def snapshot_get_branches(
-        self, snapshot_id, branches_from=b"", branches_count=1000, target_types=None
-    ):
+        self,
+        snapshot_id: Sha1Git,
+        branches_from: bytes = b"",
+        branches_count: int = 1000,
+        target_types: Optional[List[str]] = None,
+    ) -> Optional[Dict[str, Any]]:
         """Get the content, possibly partial, of a snapshot with the given id
 
         The branches of the snapshot are iterated in the lexicographical
         order of their names.
 
         Args:
-            snapshot_id (bytes): identifier of the snapshot
-            branches_from (bytes): optional parameter used to skip branches
+            snapshot_id: identifier of the snapshot
+            branches_from: optional parameter used to skip branches
                 whose name is lesser than it before returning them
-            branches_count (int): optional parameter used to restrain
+            branches_count: optional parameter used to restrain
                 the amount of returned branches
-            target_types (list): optional parameter used to filter the
+            target_types: optional parameter used to filter the
                 target types of branch to return (possible values that can be
                 contained in that list are `'content', 'directory',
                 'revision', 'release', 'snapshot', 'alias'`)
+
         Returns:
             dict: None if the snapshot does not exist;
               a dict with three keys otherwise:
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -742,19 +742,24 @@
 
     @timed
     @db_transaction_generator()
-    def snapshot_missing(self, snapshots, db=None, cur=None):
+    def snapshot_missing(
+        self, snapshots: List[Sha1Git], db=None, cur=None
+    ) -> Iterable[Sha1Git]:
         for obj in db.snapshot_missing_from_list(snapshots, cur):
             yield obj[0]
 
     @timed
     @db_transaction(statement_timeout=2000)
-    def snapshot_get(self, snapshot_id, db=None, cur=None):
-
+    def snapshot_get(
+        self, snapshot_id: Sha1Git, db=None, cur=None
+    ) -> Optional[Dict[str, Any]]:
         return self.snapshot_get_branches(snapshot_id, db=db, cur=cur)
 
     @timed
     @db_transaction(statement_timeout=2000)
-    def snapshot_get_by_origin_visit(self, origin, visit, db=None, cur=None):
+    def snapshot_get_by_origin_visit(
+        self, origin: str, visit: int, db=None, cur=None
+    ) -> Optional[Dict[str, Any]]:
         snapshot_id = db.snapshot_get_by_origin_visit(origin, visit, cur)
 
         if snapshot_id:
@@ -764,20 +769,22 @@
 
     @timed
     @db_transaction(statement_timeout=2000)
-    def snapshot_count_branches(self, snapshot_id, db=None, cur=None):
+    def snapshot_count_branches(
+        self, snapshot_id: Sha1Git, db=None, cur=None
+    ) -> Optional[Dict[str, int]]:
         return dict([bc for bc in db.snapshot_count_branches(snapshot_id, cur)])
 
     @timed
     @db_transaction(statement_timeout=2000)
     def snapshot_get_branches(
         self,
-        snapshot_id,
-        branches_from=b"",
-        branches_count=1000,
-        target_types=None,
+        snapshot_id: Sha1Git,
+        branches_from: bytes = b"",
+        branches_count: int = 1000,
+        target_types: Optional[List[str]] = None,
         db=None,
         cur=None,
-    ):
+    ) -> Optional[Dict[str, Any]]:
         if snapshot_id == EMPTY_SNAPSHOT_ID:
             return {
                 "id": snapshot_id,